The purpose of these studies is to collect basic data on intraspecific aggression (agonistic behavior) in female mice (Mus musculus). Artificial selection for isolation-induced, interfemale aggression, beginning with a wild-trapped population, has progressed through generation S11. Selection is single-sex, within families, with replicated High, unselected Control, and Low lines. Good separation among the lines has been achieved. A recent study documented the fact that maternal (postpartum) aggression has shown a correlated response to selection for isolation-induced aggression. There is no evidence, however, to indicate that male aggression has shown a correlated response. Another recent study demonstrated estrous-cycle fluctuations in aggression, with aggressiveness being high at proestrus and metestrus and low at estrus and diestrus. The proposed work for the coming year includes (1) continuation of the artificial selection program; (2) continuation of testing of males to determine whether they have shown a correlated response; (3) assaying the endogenous levels of progesterone; testosterone, and estradiol of females in the selection lines; (4) study the maternal behavior of the lines; and (5) determine the relative reproductive fitness of the lines, i.e., do aggressive females have a reproductive advantage?